Simple imaging systems used today are generally two dimensional. A camera has a single image path, or optical path and produces a two dimensional image. The term "camera" used throughout the specification means any type of single lens imaging system including a single imaging sensor which can produce or reproduce a picture of an object. Such cameras can operate in a wide frequency range extending from sonic frequencies to radio frequencies. Examples of such imaging systems include, but are not limited to, video cameras, film cameras, ultrasound systems and radio antennas.
Stereoscopic optical systems that produce three dimensional views are known. A majority of these systems include two separate cameras that provide separate side by side images and a method of blanking out alternate images from a left and right camera so the viewer sees the alternate images with a left eye followed by a right eye. These systems include active eyewear wherein shuttering occurs at the eyewear itself for viewing a monitor, or passive eyewear where the shuttering occurs at the monitor. In the case of video images, the monitor is frequently a standard 120 Hz monitor, therefore the emitter signals are synchronized to shutter alternate left and right lenses of the eyewear quickly at 120 Hz, the same speed as the monitor.
One use to which three dimensional imaging is now being used is minimal access surgery. In the known systems today dual lenses are provided in an endoscope with left and right cameras to provide left and right images for viewing. A description of existing systems are disclosed in a publication entitled "Three Dimensional Endoscopic Imaging for Minimal Access Surgery" by Mitchell et al, published October 1993, J. R. Coll. Surg. Edinb.
In two dimensional optical lens systems the rays from an object pass through entry lenses and are limited either by the edges of the lenses or an aperture stop within the lens system. This limitation of the rays is called the entry stop or entry aperture and defines the size of what is referred to as the entry pupil. The location of the entry pupil may be in front of the entry aperture or behind it. The entry pupil limits the angular aperture of the cone of rays that traverses the lens system. The exit pupil is the image of the entry aperture formed by all the parts of the lens system including the exit lenses that come after the entry aperture. The entry pupil and the exit pupil occupy conjugate positions, i.e., are coupled or twinned, with regards to the complete lens system. The principal ray from any object point is the one which passes through the center of the entry pupil.
If smaller aperture stops are placed within the lens system after the entry pupil, the complete cone of rays from the entry lens may not pass through the smaller stops. This effect, known as vignetting (i.e., partial occlusion), reduces the illumination of the image but does not change the angle of view passing through the entry pupil. This description of optical lens is disclosed by L. C. Martin in Geometrical Optics, published 1956 by Philosophical Library, Inc. (pages 71,72).
When a camera is coupled to the exit pupil, the rays defining the exit pupil are focused by the camera focusing lens to form an image on the light sensitive element in the camera. This is known as a two dimensional system, which has a single image path and applies to all types of two dimensional scopes.
In order to produce three dimensional or stereoscopic viewing, attempts have been made to place a shutter over half the entry aperture of the entry pupil and move the shutter from side to side. An example of such a system is disclosed by Lia in U.S. Pat. No. 5,222,477 which shows an endoscope, and by McElveen in U.S. Pat. No. 4,303,316 which relates to a process for recording visual scenes for reproduction in stereopsis. Krueger in U.S. Pat. No. 4,568,160 also shows inserting a refracting member between lenses. All of these systems shift the principal ray from the center of the entry pupil to one side. Thus, different perspectives, i.e., a left perspective and a right perspective, are produced which can provide stereoscopic viewing. The resulting image loses one F stop of light as the aperture stop of the entry pupil only has half the original area.
By placing a shutter in the lens system, or between the lens system and the object, it is generally required to design a special lens system for this purpose. As shown by Lia a special endoscope has to be constructed with the shutter at the external end of the scope.
Other methods of three dimensional viewing do not use a single image system. Various combinations of additional prisms, mirrors, refracting lenses have been positioned either in the entry pupil or the exit pupil. In all these cases two focusing lenses are needed instead of one as there are two separate sets of image rays or image paths that create two separate images.
Other types of stereoscopic optical systems are disclosed in U.S. Pat. No. 4,761,066 to Carter which utilizes a beam splitter. With regard to the viewers, an example of a liquid crystal stereoscopic viewer is disclosed by Roese in U.S. Pat. No. 4,021,846. The concept of utilizing a passive eyewear includes lenses with colored filters therein. Such a system is disclosed in U.S. Pat. No. 3,712,199 to Songer.